A primary use case is why a user uses a product.

A secondary use case is just a bonus.

But some secondary use cases have a network effect.

Their value goes up super-linearly with use and adoption.

This can allow the secondary use case to quickly eclipse the original primary use case.

The primary use case typically must work on its own without a network effect so the very first users get sufficient value to use it even when no one else does yet.

A recipe for a hyper-viral product:

When users do their primary use case, they leave byproducts in the system that are an input to the secondary use case.

The secondary use case has combinatorial value that scales with the amount of byproducts the user left.

The secondary use case can be activated proactively with zero friction.

As more users use the product, they create huge amounts of zero-friction adjacent use cases of self-ratcheting quality.